This application is based on Japanese Patent Application No. 10-320959 filed on Nov. 11, 1998, the content of which is hereby incorporated by reference.
1. Field of the Invention
This invention relates to a liquid crystal light modulating device using a liquid crystal material interposed between a pair of substrates, and a method for manufacturing same.
2. Description of Related Art
Conventional light modulating devices are widely known which interpose a liquid crystal material between two substrates and use the liquid crystal material as a light shutter. This type of light modulating device is used, for example, as a display device. This type of display device is used as a display in various devices, foremostly, for example, in notebook style personal computer displays and portable telephone displays. Liquid crystal display devices characteristically are thin in design and have low power consumption, and are widely used in portable devices having a small surface area. In recent years, however, such displays have been developed for large screen displays used in built-in in-wall televisions and as replacements for CRT monitors.
Maintaining a uniform gap between substrates has become more important in conjunction with achieving higher quality and larger surface area liquid crystals. Heretofore, it has been difficult to regulate sufficient uniformity of the inter substrate gap within a light modulation range, with resultant disadvantages including display defects which make it difficult to achieve high quality displays. Management of the flatness of the inter substrate gap is directly concerning with a parameter which defines the display quality particularly in liquid crystal display devices using ferrodielectric liquid crystals, and reflective type liquid crystal display devices using liquid crystals which exhibits a cholesteric phase (i.e., chiral nematic liquid crystals). In ferrodielectric liquid crystals, irregularities in the inter substrate gap cause irregularities in the orientation of the liquid crystals which reduces the display quality. When driving an element which has a memory such as cholesteric liquid crystals, the display irregularities become pronounced due to the reduced threshold characteristics.
Art related to controlling the inter substrate gap is known, e.g., U.S. Pat. No. 4,249,800. This United States patent discloses a liquid crystal display which uses glass beads coated with a thermoplastic resin as spacers. Specifically, the glass beads are dispersed in a photosensitive lacquer, and applied on a substrate, then the glass beads are arranged on the substrate by a so-called photolithographic method which includes masked light exposure and developing.
In the method described in this United States patent, first, manufacturing or preparation of the resin-coated glass beads which requires a labor is necessary. Furthermore, a large amount of the spacers are wastefully removed from the substrate during the photolithographic method. In addition, productivity is low due to the extreme complexity of the various processes such as the required developing process. Since only part of the surface coating layer of the spacers comes into contact with the substrate, adequate adhesion cannot be obtained. Since the spacers themselves are small it is difficult to accurately arrange these spacers at optional positions, essentially making it difficult to eliminate display irregularities.
U.S. Pat. No. 5,285,304 discloses a ferroelectric liquid crystal display in which two substrates, on which adhesive thermosetting polymer particles and nonadhesive thermoplastic polymer particles having a diameter approximately 1.5xcx9c5 times the thickness of the liquid crystal layer are dispersed, are pressed each other. Japanese Laid-Open Patent Application No. 8-110524 discloses a method wherein a gap control member which is not deformable even when heated and a gap control member supplying adhesive force to both substrates when hardened after being melted or softened by heating are applied on substrates, and the latter gap controller adheres to both substrates when melted or heated above the softening temperature.
In the methods disclosed in the aforesaid two publications, it is essentially impossible to arrange the gap controller or polymer particles adhering to the substrate at a desired position because, the polymer particles or gap controller are arranged dispersion. Accordingly, when dispersing these materials, the polymer particles or gap controllers easily start to flocculate. As a result, the locations of such flocculation readily produce defects in the orientation of the liquid crystal, and produce regions where light is unmodulated such that a display of sufficiently high quality cannot be obtained.
In contrast, Japanese Laid-Open Patent Application No. 9-258233 proposes a liquid crystal display device wherein spacers are adhered to a pair of substrates by an ultraviolet-curing resin. The arrangement of the spacers on the substrates is accomplished by a silkscreen printing method wherein the spacers are mixed in an ultraviolet-curing resin beforehand. Thereafter, the opposite substrate is overlaid and the ultraviolet-curing resin is hardened by exposure to ultraviolet light. However, sufficient adhesion force is not obtained even when both substrates are adhered inasmuch as the cross section area of the spacers is too small. Furthermore, in general, ultraviolet-curing resin is soft when in an uncured state, such that when the two substrates are overlaid, even a slight dislocation of the substrates may move the oriented spacers and the ultraviolet-curing resin. At this time the ultraviolet-curing resin broadens the interface between the liquid crystal and the substrates so as to cause orientation defects of the liquid crystal in the regions of the broadened ultraviolet-curing resin.
Japanese Laid-Open Patent Application No. 62-203123 discloses a liquid crystal display device using high polymer resin dams continuously arrayed in a matrix across the entire surface of a transparent flexible substrate rather than the particle-like spacers of the conventional art. The purpose of forming resin dams described in this publication is to adhere the resin dams to as long flexible substrate, and cut them to an optional size. According to this publication, an adhesion force is generated between the substrates and the resin damns under light pressure; specifically, an example is given wherein the resin dams are formed by an offset printing method using ultraviolet-curing resin, or photolithographic method using a photoresist, and adhered to the substrates under light pressure at approximately 80xc2x0 C.
Even when the resin dams are formed in a matrix pattern, for example, it is difficult to realize an accurate inter substrate gap only by the resin dams. In the aforesaid example, since the formed resin structure itself comprises UV-curing resin, sufficient adhesion is not achieved by light pressure on the substrates at approximately 80xc2x0 C. Accordingly, there is high concern of inter substrate gap fluctuation due to rapid temperature change or external pressure, which will disadvantageously prevent proper light modulation by the liquid crystal layer.
From the perspective of imparting adhesion to the resin structure, a method is proposed wherein the resin structure is formed which is in contact with a pair of substrates using a photopolymerizable material. For example, U.S. Pat. No. 5,473,450 discloses a liquid crystal display device wherein a resin wall is formed by photopolymerization phase separation using a photomask. U.S. Pat. No. 5,682,218 discloses resin columns formed by separating the liquid crystal and uncured resin by cooling a mixture of liquid crystal in isotropic phase and resin (monomer), and thereafter heating or UV-curing the uncured resin.
In methods using photopolymerizable materials, uncured monomer and polymerization initiator may remain within the liquid crystal even after polymerization, thereby disadvantageously affecting the operating characteristics and contrast of the liquid crystal. Furthermore, irregularities readily occur in the constituents of the resin structure itself so as to affect the inter substrate gap.
On the other hand, vacuum injection methods are used in methods for manufacturing liquid crystal display devices. In such manufacturing methods, first, a resin seal is formed so as to provide an opening for injecting liquid crystal material on the edge of one substrate among a pair of glass substrates provided with attached electrodes, and spacers are dispersed on the other substrate to maintain a spacing (gap of predetermined size between the substrates. Thereafter, both substrates are adhered and heated to cure the resin seal and produce the panel. This panel is placed in a container under reduced pressure, to produce a vacuum within the panel with the liquid crystal material touching the opening. Finally, the liquid crystal material is injected into the panel by returning the interior of the container to normal pressure.
Since the injection device must be enlarged and the injection time lengthened in the aforesaid vacuum injection method when the display surface area is large, a more efficient sealing method was determined to replace the aforesaid method.
Countermeasures to eliminate the previously described disadvantages are disclosed in Japanese Laid-Open Patent Application Nos. 61-190313 and 9-127528. The liquid crystal display manufacturing methods described in these publications first form a resin seal on a substrate, drip liquid crystal material onto the substrate, press another substrate to adjust a desired inter substrate gap, and subsequently harden the resin seal. Accordingly, the previously mentioned vacuum injection method is not required.
These published methods include the following disadvantages, however. That is, the method disclosed in Japanese Laid-Open Patent No. 61-190313 provides that the resin seal is cured when the substrates are pressed together to form a uniform gap therebetween. The liquid crystal material dropped inside the resin seal have fluidity, and since the resin seal is in the uncured stage there may be dislocation of the substrate when pressure is applied which may remain even when the resin seal is later cured, thus presenting another disadvantage to those occurring when the vacuum injection method is used. Japanese Laid-Open Patent No. 9-127528 proposes a method using a thermoplastic photosetting re(sin having a softening point between room temperature and the N-I point of the liquid crystal material. The N-I point of the liquid crystal material is approximately 100xc2x0 C. In this way material having a softening point at a low temperature make possible the softening of the resin seal under normal usage conditions due to heat given off by backlighting, or the closed interior of a room or automobile. Accordingly, the softened resin seal component within the liquid crystal material may elute, such that the softened resin seal forms a thin film on the interface between the liquid crystal material and the substrate, thereby reducing the reliability of the display and causing defects in the orientation of the liquid crystals.
In view of the previously described disadvantages, an object of the present invention provides a liquid crystal light modulating device capable of controlling a uniform inter substrate gap to provide a high quality display, and a method for manufacturing same.
Another object of the present invention is to provide a liquid crystal light modulating device capable of efficiently and simply sealing liquid crystal material between substrates, and a method for manufacturing same.
At least one of these objects are attained by the liquid crystal light modulating device reflecting one of aspects of the present invention. The liquid crystal light modulating device comprises a pair of substrates at least one of which is a transparent substrate, liquid crystal material interposed between the pair of substrates, spacers for maintaining a predetermined gap between the pair of substrates, and resin structural members arranged in a predetermined arrangement within the display region, and wherein each of said resin structural members is made from a curable resin material as a main component, and forms a macromolecular material having thermoplasticity after curing, and which adheres between the pair of substrates. A resin seal may also be formed at the periphery of the pair of substrates.
The method for manufacturing the liquid crystal display device reflecting one of aspects of the present invention comprises the steps of: producing a plurality of resin structural members by arranging a macromolecular material having a curable resin material as a main component to a thickness greater than a predetermined inter substrate gap on at least one substrate among a first and a second substrate; supplying a liquid crystal material to the surface of at least one substrate among the first and the second substrates; and applying pressure on the first and the second substrates via a pressure member and heating same to adjust the inter substrate gap to a predetermined gap and to adhere the pair of substrates. In addition to these steps, steps of; arranging a resin seal on the periphery of either the first or the second substrate; and curing the resin seal via exposure to light, may be comprised. The other of the substrates used may have a plasticity. A step may be provided to arrange spacers on at least one of the substrates. Adjustment of the inter substrate gap is more readily accomplished by arranging the spacers. When the spacers are mixed beforehand with the liquid crystal material, the liquid crystal material including the spacers may be supplied to the substrate surface.
In the present specification, the part accomplishing light modulation in the liquid crystal light modulation layer interposed between the substrates is deemed the light modulation region. For example, in FIGS. 3 and 4 discussed later, the part circumscribed by the resin seal 26 is deemed as the light modulation region. When a liquid crystal light modulation device is used as a liquid crystal display device, the light modulation region works as a dislay region.
According to the present invention, a uniform inter substrate gap is maintained by supporting a pair of substrates via a resin structure adhered to the top and bottom pair of substrates. Using a material having a curable resin material as a main component as the resin structure reduces elution of impurities to the liquid crystal material, and suppresses the degradation of optical characteristics of the display element.
Particularly if the curable resin material is a photosetting type, the liquid crystal material may be dripped onto the substrate after the resin material is cured via exposure to light. This process reduces staining by impurities from the uncured resin structure material within the liquid crystal material. This process further reduces the period of tackiness and improves production efficiency. Heating the resin structural material while applying pressure provides excellent adhesion of the resin structure to both substrates and supports both substrates. Simultaneously injecting and sealing the liquid crystal material and adhering the substrates provides marked improvement of productivity.
Even the use of a thermosetting resin as the curable resin material suppresses elution of impurities to the liquid crystal material. When an electron beam type curing resin is used, polymerization can be achieved without including a polymerization initiator in the resin material, thereby suppressing elution of impurities to the liquid crystal material.
Overlaying the two substrates while the resin seal is in a semi-cured state prevents elution of the resin seal component into the liquid crystal material even when the resin seal material contacts the liquid crystal material. When the resin seal has a ring-like structure, air is not suctioned between the two substrates even when the substrates expand after pressure is applied via a pressure member. When the resin seal is provided on a different substrate than the substrate provided with a resin structure and bot substrates are adhered, heat may be applied to the semi-cured resin seal only when pressure is applied, thereby suppressing a reduction in the adhesion force caused by over curing of the resin seal
In the present invention, the substrate may be easily anchored by vacuum suction of one substrate on a base, so as to reliably prevent movement of the substrate due to the friction force generated when the flat plate of pressure member is moved. If a substrate is heated while on a flat plate, the fluidity of the liquid crystal material increases, so as to not only reduce enfolding of air bubbles in the liquid crystal material, but also accelerate softening of the adhesive materials (resin seal, resin structure) to allow adhesion of both substrates in a short period.
A roller shaped pressure member is advantageous to handle, and when a heatable roller type member is used, the adhesive material can be softened by heating the substrate, thereby improving the adhesion force of both substrates.